SIMS zircon U‐Pb dating from bentonites in the Penglaitan Global Stratotype Section for the Guadalupian–Lopingian boundary (GLB), South China

The Penglaitan section, as the Global Stratotype Section for the Guadalupian–Lopingian boundary (GLB), displays continuous deposition with a complete succession of pelagic conodont zones across the GLB. However, there is no reliable radiometric age from the Penglaitan section itself to constrain the GLB. Here, we report SIMS zircon U‐Pb ages from two bentonite layers (Bed 7c) in the Penglaitan Global Stratotype Section near the GLB. The sample PL‐62‐1 yields a weighted mean ²³⁸U/²⁰⁶Pb age of 257.1 ± 2.2 Ma, and the sample PL‐62‐2 yields a weighted mean ²⁰⁶Pb/²³⁸U age of 257.0 ± 4.2 Ma. Therefore, we consider 257.0–257.1 Ma as the age of deposition of Bed 7c (the end of the C. postbitteri postbitteri conodont Zone, ca. 86 cm above the GLB), and, considering the depositional rate of chert, we suggest 258.6 Ma as the age of the GLB. Copyright © 2015 John Wiley & Sons, Ltd.     

Petrogenesis of the Yangchang Mo‐bearing granite in the Xilamulun metallogenic belt,NE China: geochemistry,zircon U–Pb ages and Sr–Nd–Pb isotopes

The Yangchang granite‐hosted Mo deposit is typical of the Xilamulun metallogenic belt, which is one of the important Mo–Pb–Zn–Ag producers in China. A combination of major and trace element, Sr, Nd and Pb isotope, and zircon U–Pb age data are reported for the Yangchang batholith to constrain its petrogenesis and Mo mineralization. Zircon LA‐ICPMS U–Pb dating yields mean ages of 138 ± 2 and 132 ± 2 Ma for monzogranite and granite porphyry, respectively. The monzogranites and granite porphyries are calc‐alkaline with K₂O/Na₂O ratios of 0.75–0.92 and 1.75–4.42, respectively. They are all enriched in large‐ion lithophile elements (LILEs) and depleted in high‐field‐strength elements (HFSEs) with negative Nb and Ta anomalies in primitive‐mantle‐normalized trace element diagrams. The monzogranites have relatively high Sr (380–499 ppm) and Y (14–18 ppm) concentrations, and the granite porphyries have lower Sr (31–71 ppm) and Y (5–11 ppm) concentrations than those of monzogranites. The monzogranites and granite porphyries have relatively low initial Sr isotope ratios of 0.704573–0.705627 and 0.704281, respectively, and similar ²⁰⁶Pb/²⁰⁴Pb ratios of 18.75–18.98 and 18.48–18.71, respectively. In contrast, the ε_Nd(t) value (−3.7) of granite porphyry is lower than those of monzogranites (−1.5 to −2.7) with Nd model ages of about 1.0 Ga. These geochemical features suggest that the monzogranite and granite porphyries were derived from juvenile crustal rocks related to subduction of the Paleo‐Pacific plate under east China. Copyright © 2013 John Wiley & Sons, Ltd.     

New high‐resolution age data from the Ediacaran–Cambrian boundary indicate rapid,ecologically driven onset of the Cambrian explosion

The replacement of the late Precambrian Ediacaran biota by morphologically disparate animals at the beginning of the Phanerozoic was a key event in the history of life on Earth, the mechanisms and the time‐scales of which are not entirely understood. A composite section in Namibia providing biostratigraphic and chemostratigraphic data bracketed by radiometric dating constrains the Ediacaran–Cambrian boundary to 538.6–538.8 Ma, more than 2 Ma younger than previously assumed. The U–Pb‐CA‐ID TIMS zircon ages demonstrate an ultrashort time frame for the LAD of the Ediacaran biota to the FAD of a complex, burrowing Phanerozoic biota represented by trace fossils to a 410 ka time window of 538.99 ± 0.21 Ma to 538.58 ± 0.19 Ma. The extremely short duration of the faunal transition from Ediacaran to Cambrian biota within less than 410 ka supports models of ecological cascades that followed the evolutionary breakthrough of increased mobility at the beginning of the Phanerozoic.     

SHRIMP U–Pb zircon ages of pyroclastic rocks in the Bansong Group, Taebaeksan Basin, South Korea and their implication for the Mesozoic tectonics

The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two well-known orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3 ± 1.5 and 187.2 ± 1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic. Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages.The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergence developed in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks, but an “intracontinental” thrust that developed after their juxtaposition.     

Evolution of the Yunkai Terrane, South China: Evidence from SHRIMP zircon U–Pb dating, geochemistry and Nd isotope

The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains 242 Ma metamorphic zircons, as well as 440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t_DM model ages of 2.17–1.91 Ga and ε_Nd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t_DM model ages of 2.13–1.42 Ga and ε_Nd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.     

Internal structures and U–Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.     

Two episodes of the Indosinian thermal event on the South China Block: Constraints from LA-ICPMS U–Pb zircon and electron microprobe monazite ages of the Darongshan S-type granitic suite

The Darongshan granitic suite (~ 10,000 km²) consists of five major units (Taima, Nadong and Jiuzhou plutons, and Pubei and Darongshan batholiths) typical of peraluminous S-type granitoids containing abundant granulite inclusions in the Cathaysia block, South China. Six samples from these plutons and batholiths have been investigated using both LA-ICPMS U–Pb age dating on zircon cores and EMP U–Th–Pb chemical age dating on monazite cores and rims. LA-ICPMS zircon results give similar major age populations ranging between 260 ± 3 and 250 ± 3 Ma for all units, with apparent older age peaks concentrated at 1020, 800, 430 and 330 Ma. On the other hand, EMP monazite results yield younger ages of 231–229 Ma for Nadong, Taima, Pubei and Darongshan and 224 Ma for Jiuzhou samples, with older age groups of 264 Ma for Taima and 256–250 Ma for Pubei units. Since the older monazite ages are similar to the majority of zircon ages, the latter are considered as inherited ages. Further because such zircon ages are similar with the emplacement time of the Emeishan large igneous province in western South China, they likely reflect the timing of metamorphism for the included fragments of granulitic crusts that had been formed by invasion of the Emeishan plume. The younger monazite ages, as present for all plutons and batholiths in the entire Darongshan area, are taken as the formation age of the host granites. Combining U–Pb zircon and EMP monazite ages known for Permo-Triassic high temperature and high pressure metamorphic rocks and granites in the Indochina block (e.g., the Kannack Complex of the Kontum massif), it is suggested that the Indosinian thermal activity had set records over both the Indochina (plus Simao) and South China blocks in two main episodes, one is 260–250 Ma and the other is 231–229 Ma. One plausible explanation is that these two blocks were one united continent before the Emeishan plume activity and an opening was triggered by this plume at ~ 260 Ma. Due to forces of the approaching Sibumasu block, both the South China and Indochina blocks were amalgamated again at ~ 230 Ma. We, therefore, advocate that double subduction of the plume-triggered oceanic crusts in opposite directions is responsible for the generation of the Darongshan granitic suite in the South China block and its counterpart in the Indochina block.     

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.     

U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh‐pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction‐zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti‐in‐zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction‐zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti‐in‐zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr‐in‐titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr‐in‐titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction‐zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid‐T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.     

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

The provenance of Western Irish Namurian Basin sedimentary strata inferred using detrital zircon U–Pb LA‐ICP‐MS geochronology

The Western Irish Namurian Basin (WINB) preserves classic examples of basin floor sequences through to slope deposits and deltaic cyclothems. Despite over 50 years of research into the WINB, its sediment provenance remains highly contested. Sedimentological arguments, including palaeocurrent vectors and palaeoslope indicators have been invoked to propose a sediment source from the NW or the west (i.e. from within Laurentia). These same indicators have been subsequently reinterpreted to reflect a southern provenance. It is not clear from sedimentological arguments alone which interpretation more accurately reflects the infilling of the WINB. Regional‐scale constraints on WINB provenance may be obtained with detrital zircon U–Pb geochronology. U–Pb LA‐ICP‐MS detrital zircon analysis was undertaken on samples from three sandstone units at different stratigraphic levels within the WINB siliciclastic sedimentary fill (Ross Formation, Tullig Sandstone, Doonlicky Sandstone). The samples are dominated by 500–700 Ma zircons, which can be correlated with Cadomian–Avalonian orogenic activity within terranes to the south of the WINB (Avalonia/Ganderia, Armorica and Iberia). In contrast, Eastern Laurentia, to the north of the WINB, was devoid of orogenic activity at this time. WINB samples also yield age populations younger than 500 Ma, and older than 700 Ma. These are not diagnostic of a particular source terrane and thus could be derived from terranes north and/or south of the WINB. WINB detrital zircon age spectra can be reconciled by an Avalonian or combined Avalonian–Laurentian provenance for WINB sedimentary strata. Further research is required in order to distinguish between these two possibilities. Copyright © 2011 John Wiley & Sons, Ltd.     

Tectonic implications of new U–Pb zircon ages of the Ladakh batholith,Indus suture zone,northwest Himalaya,India

We determined U–Pb ages on zircons from Ladakh granitoid samples of three previously undated plutons and deduced four distinct age groups between c. 67 and c. 45 Ma (66.6 ± 2.1, 57.6 ± 1.4, 53.4 ± 1.8, 52.50 ± 0.53 and 45.27 ± 0.56 Ma). This suggests that the Ladakh batholith grew by addition of at least four distinct subduction‐related magma pulses at c. 67, 58, 53 and 45 Ma, thus indicating that the belt was continuously active throughout the Palaeocene and the Middle Eocene (Lutetian). The 45.27 ± 0.56 Ma pluton at Daah‐Hanu is the last major calcalkaline arc magmatic pulse in the Ladakh batholith. Thereafter, the subduction‐related major plutonism gradually waned. The earlier estimate for the youngest pluton within the Ladakh batholith is 49.8 ± 0.8 Ma for the Leh pluton ( J. Geol., 2000, 108 , 303 ).     

Dating pseudotachylyte of the Asuke Shear Zone using zircon fission-track and U–Pb methods

Zircon fission-track (FT) and U–Pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the Asuke Shear Zone (ASZ), Aichi Prefecture, Japan. The U–Pb ages of all four samples are  67–76 Ma, which is interpreted as the formation age of Ryoke granitic rocks along the ASZ. The mean zircon FT age of host rock is 73 ± 7 (2σ) Ma, suggesting a time of initial cooling through the zircon closure temperature. The pseudotachylyte zone however, yielded a zircon FT age of 53 ± 9 (2σ) Ma, statistically different from the age of the host rock. Zircon FTs showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone. Coupled with the new zircon U–Pb ages and previous heat conduction modeling, the present FT data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault. The age for the pseudotachylyte coincides with the change in direction of rotation of the Pacific plate from NW to N which can be considered to initialize the NNE–SSW trending sinistral–extensional ASZ before the Miocene clockwise rotation of SW Japan. The present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon FT thermochronology.     

Structure and U–Pb zircon geochronology in the Variscan foreland of SW Sardinia,Italy

New field mapping, U–Pb zircon geochronology and structural analysis of the southernmost Sardinia metamorphic basement, considered a branch of the Variscan foreland, indicate that it is, in part, allochthonous and was structurally emplaced within the foreland area, rather than being older depositional basement beneath the foreland succession. The Bithia Formation, classically considered part of the ‘Southern Sulcis metamorphic Complex’ (and here termed the Bithia tectonic unit, or BTU), is a greenschist facies metamorphic unit commonly interpreted as Precambrian in age. New geochronology of felsic volcanic rocks in the BTU, however, yield a U–Pb zircon age of 457.01 ± 0.17 Ma (Upper Ordovician). Thus, the depositional age of the unit is younger than the weakly metamorphosed Lower Cambrian rocks of the adjacent foreland succession. New detailed mapping and analysis of the field relationships between the BTU and foreland succession indicates that their contact is a mylonitic shear zone. The metamorphic character, general lithology, and deformational history of the BTU are similar to those of units in the Variscan Nappe Zone located northeast of the foreland area. We reinterpret the BTU as a synformal klippe of material related tectonically to the Variscan Nappe Zone. We infer that it was thrust over and became infolded into the foreland during late stages of the Variscan contractional deformation. Copyright © 2012 John Wiley & Sons, Ltd.     

Detrital zircon U–Pb provenance and palaeogeography of Triassic rift basins in the Marrakech High Atlas

The Marrakech High Atlas contains some of the best exposures of the Triassic early‐rift strata related to Atlantic opening in NW Africa. We present the first detrital zircon U–Pb data of five Triassic redbed samples from the Tizi n'Test basin to quantify sediment provenance, transport and dispersal patterns during early rifting. These U–Pb ages document dominant sediment sourcing from the south, the Anti‐Atlas domain, with very limited to absent input from the Variscan Meseta domain to the north. This combined with stratigraphic and thermochronologic information points to a highly asymmetric palaeogeography during Triassic rifting. Furthermore, the occurrence of Archaean detrital zircon grains in Triassic sandstone, likely recycled from the Reguibat shield, suggests the presence of a fully developed regional drainage system with rivers and catchments reaching hundreds of kilometres into the hinterland of the rift flank.     

Zircon U–Pb SHRIMP ages of weakly to unmetamorphosed granitoids of the Yangtze basement outcrop in Dabieshan, central China

The Qichun granitoids exposed in the Dabie Orogen of China are composed of two types of rocks: porphyritic monzogranite (with variable schistosity) and syenogranite (without schistosity). The two types show large differences in geochemical characteristics. The porphyritic monzogranite is characterized by high Al₂O₃ content (15.73%), relatively high CaO (2.46%) and Na₂O contents (Na₂O/K₂O=1.27), strong depletion in HREE and strong fractionation between LREE and HREE ((La/Yb)_N=46.8), similar to some high Al₂O₃ Archaean TTG gneisses. Conversely, the syenogranite is characterized by relatively low Al₂O₃ (14.05%) and CaO (0.82%) contents, and higher K₂O than Na₂O (Na₂O/K₂O=0.81). The degree of fractionation between LREE and HREE is minor. The U–Pb SHRIMP zircon age of the porphyritic monzogranite is 841±15 and 824±27 Ma for the syenogranite. These ages are similar to the protolith emplacement ages of granitic gneisses in the Dabie Orogenic Belt. The existence of weakly to unmetamorphosed granitoids in the Dabie Orogen shows that the granitoids were situated in the back part of the subducted plate during collision and subduction between the Yangtze and the North China cratons, and may represent outcrops of the Yangtze basement.     

Zircon U–Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China

The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751 ± 7 Ma for protolith crystallization, and two group ages of 213 ± 4 to 245 ± 17 Ma and 126 ± 4 to 131 ± 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative ε_Hf(t) values of − 5.1 to − 2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive ε_Hf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ¹⁸O values of − 3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ¹⁸O rocks. North Dabie shares the same age of Neoproterozoic low δ¹⁸O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.     

Sr isotope excursion across the Precambrian–Cambrian boundary in the Three Gorges area, South China

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life. However, the scarcity of well-preserved outcrops across the boundary leaves an obstacle in decoding surface environmental changes and patterns of biological evolution.In south China, strata through the PC/C boundary are almost continuously exposed and contain many fossils, suitable for study of environmental and biological change across the PC/C boundary. We undertook deep drilling at four sites in the Three Gorges area to obtain continuous and fresh samples without surface alteration and oxidation. ⁸⁷Sr/⁸⁶Sr ratios of the fresh carbonate rocks, selected based on microscopic observation and geochemical signatures of Mn/Sr and Rb/Sr ratios, aluminum and silica contents, and δ¹³C and δ¹⁸O values, were measured with multiple collector-inductively coupled plasma–mass spectrometric techniques.The chemostratigraphy of ⁸⁷Sr/⁸⁶Sr ratios of the drilled samples displays a smooth curve and a large positive anomaly just below the PC/C boundary between the upper part of Baimatuo Member of the Dengying Formation and the lower part of the Yanjiahe Formation. The combination of chemostratigraphies of δ¹³C and ⁸⁷Sr/⁸⁶Sr indicates that the ⁸⁷Sr/⁸⁶Sr excursions preceded the δ¹³C negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO₂, caused biological depression.     

Detrital footprint of the Mozambique ocean: U–Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar

The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between 800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, 2650, 2500 and 800–700 Ma.The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.